Low density high strength steel and method for producing said steel

ABSTRACT

A low density high strength steel sheet including 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al and 0.01% or less N, the balance being iron and inevitable impurities, wherein 1.7·(Mn—Al)+52.7·C is at least 3 and at most 4.5. A method of producing the low density and high strength steel sheet.

The present invention relates to a low density and high strength steel sheet and to a method of producing a low density and high strength steel sheet, for instance for use as inner or outer steel sheets of a structural member for an automobile.

Because of its excellent strength and ductility, and very low cost as compared to aluminium or magnesium, steel has been generally used to make a body of the automobile lighter by developing stronger grades which allow the use of thinner high strength steel sheet. However, in order to overcome a future limitation to the reduction in weight, it may be required to use alternative sources for reducing the weight of steel parts.

Steels with lower density are known in the art. TWIP steels with very high manganese contents of over 20% result in a lighter steel matrix. Also aluminium, as a lightweight element, is sometimes added so as to reduce the density of the steel. Additions of up to 15% aluminium have been used.

A problem with the known steels is that processability in the existing facilities of the steel industry is problematic due to their proneness for cracking and their high deformation resistance during rolling. Other problems are weldability issues, particularly with the high aluminium content, and the likelihood of formation of undesirable martensite components.

It is an object of the invention to provide a low density and high strength steel sheet which can be produced using existing facilities of the steel industry.

It is also an object of the invention to provide a relatively low alloyed low density and high strength steel sheet which has a strength of at least 600 MPa.

It is also an object of the invention to provide a process for producing a low density and high strength steel sheet.

One or more of these objects are reached by providing a low density and high strength steel sheet comprising 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al and 0.01% or less N, the balance being iron and inevitable impurities, wherein 1.7·(Mn—Al)+52.7·C is at least 3 and at most 4.5 as claimed in claim 1. Preferable embodiments are claimed in the dependent claims.

One or more of these objects are reached by the process of claim 7. Preferable embodiments of the process are claimed in the dependent claims.

Hereinafter, the composition of the present invention will be described in detail (all compositions in weight %).

Carbon serves to create cementite (Fe,Mn)₃C and kappa carbide (Fe,Mn)₃AlC. Carbon is also an austenite stabilising element, and provides dispersion strengthening by forming precipitates. Particularly, since the columnar dendrite created during continuous casting is rapidly re-crystallized so as to create a coarse structure, the formation of carbides at high temperatures carbide is used to refine the structure. Additionally, the addition of carbon between 0.15 to 0.25% is used to increase the strength. However, if the added amount of carbon increases, the amounts of cementite and kappa carbide increase to contribute to an increase in strength, but greatly decrease ductility of steel. In the steel to which aluminium is added, particular, kappa carbide is precipitated on a grain boundary of ferrite to cause brittleness. Values below 0.15% result in too low a strength and values higher than 0.25% tends to increase the risk of welding related issues, so that the carbon content is maximised to 0.25. Preferably, the carbon content is at least 0.16%, more preferably at least 0.17%. Preferably, the carbon content is at most 0.23, more preferably at most 0.20%.

Manganese contributes to the formation of austenite at high temperature, together with carbon. Further, manganese increases the lattice constant of steel thereby decreasing the density of the steel. A minimum amount of 2.5% of manganese was found to result in stable austenite and a significant decrease in density of the steel. However, if the amount of manganese is excessive, then the occurrence of central segregation results in an excessive band structure in a hot rolled sheet, which causes a decrease in ductility. The upper limit of manganese is therefore restricted to 4%. A preferable upper limit of manganese is 3.8%.

Phosphorus is an element which is added in an amount as small as possible. It segregates on the grain boundary and causes hot shortness and cold shortness, so that workability of steel may be greatly reduced. The upper limit of phosphorus is restricted to 0.02%, but preferably the amount is limited to at most 0.01% or even 0.005%.

Similar to phosphorus, sulphur promotes hot shortness. Particularly, it creates coarse MnS, which upon hot rolling and cold rolling, may causes a rolling plate to break, so that it is limited to 0.015% or less. Preferably the sulphur amount is limited to at most 0.01% or even 0.005%.

Aluminium is an important element in the present invention, together with carbon and manganese. The addition of aluminium decreases the density of the steel. Taking into consideration the decrease in specific gravity, it is preferred that a great quantity of Al be added. The addition of 6% aluminium or more results in a significant decrease in density of the steel. However, if aluminium is excessively added, the amount of intermetallic compounds such as kappa carbide, FeAl or Fe₃Al increases which causes a reduction of the ductility of steel, leading to cracking during cold rolling, so that the upper limit is restricted to 9%. Also, aluminium causes an increase of the ductile-brittle transition temperature from sub-zero temperatures to around ambient temperatures. Therefore the upper aluminium limit is restricted to 9%. A preferable lower limit of aluminium is 6.2%.

Nitrogen causes the formation of AlN-precipitates if a great quantity of aluminium is added as in the present invention. These precipitates are effective in the refinement of columnar dendrite and the improvement in a ratio of equiaxed dendrite and for this reason a small amount of nitrogen in the steel is advantageous. However, large amounts of nitrogen cause large and amounts of potentially coarse AlN-precipitates which is undesirable. Thus, the upper limit of N is restricted to 0.01%. Preferably the nitrogen amount is limited to at most 0.008% or even 0.005%.

The inventors found that a steel within the boundaries of the chemical composition as explained hereinabove did not always result in a steel that performed satisfactorily. The ductility of the obtained sheet appeared to be too low if the outcome of the equation (1.7˜(Mn—Al)+52.7 C) is lower than 3 or higher than 4.5.

By this equation the amount of manganese and carbon is controlled in relation to the amount of aluminium in order to control the ductility.

In an embodiment the composition of the steel sheet is chosen such that the value of (36.0+Mn)/Al is at least 1.3 and at most 2.0.

In addition to the above basic composition of the present invention, in order to improve or compensate the strength, ductility, and the other physical properties of steel, small to intermediate amounts of one or two or more elements of the group consisting of Si, Cr, Mo, Ni, Cu, B, Ti, Zr, Nb, W and Ca may optionally be added.

Similar to aluminium, silicon also decreases the specific gravity of steel and contributes to the improvement in strength, but if being excessively added, it may create a thick, irregular high temperature oxide film on the surface of steel. Also, silicon causes a stronger increase of the ductile-brittle transition temperature from sub-zero temperatures to around ambient temperatures than aluminium. Therefore the upper silicon limit is restricted to 2%. Thus, it is preferred that the amount of silicon is within the range of 0.1 to 2.0%.

Chromium is a ferrite-forming element which forms Cr-based carbides which may serve to refine the microstructure, so that the amount can be 0.1% or more. However, if added too much, ductility is reduced, so that the upper limit is restricted to 0.3%.

Similar to chromium, molybdenum is a ferrite-forming element which forms fine carbide, and is added by 0.05% or more. However, if excessively added, it decreases the ductility of steel, so that the upper limit thereof is restricted to 0.5%.

Nickel is an austenite-forming element, which may introduces partial austenite during hot rolling to refine the structure, to thereby greatly improve the ridging resistibility. However, the price of nickel is high and increases the manufacturing cost, so that the limit is restricted to a range of 0.1 to 2.0%.

Copper acts similar to nickel, but generally the price of copper is lower than that of nickel, so that it can be added in the range of 0.1% or more. However, if excessively added, it exists on a grain boundary in a liquid state to cause intergranular brittleness, owing to fused metal, and causes edge cracking, so that the amount is restricted to a range of 0.1 to 1.0%.

Boron restricts the recovery and recrystallization of ferrite in the process of hot rolling so as to contribute to the structure refinement thanks to cumulative rolling reduction and increase the strength of steel, so that the amount is 0.0005% or more. However, if excessively added, it may create boron-carbide, decreases the ductility of steel, and deteriorates the wettability of a hot-dipped galvanized coating layer, so that the upper limit is restricted to 0.003%.

Titanium forms TiN or TiC or the like to thereby improve the grain refinement of the cast structure and contributes to the dispersion of kappa-carbide, so that it is added in the range of 0.01% or more. However, it is expensive and increases the manufacturing cost, and it reduces ductility due to the increase in strength through precipitation, so that the upper limit is restricted to 0.2%.

Zirconium acts similar to titanium, and forms strong nitride and carbide relative to titanium, so that it is added in the range of 0.005% or more. However, it is expensive, so that the upper limit is restricted to 0.2%.

Niobium acts similar to titanium, and thus it is added in the range of 0.005% or more. However, unlike titanium, it delays recrystallisation of the steel at high temperature to thereby greatly increase the rolling load of hot rolling. This may make it impossible to manufacture a thin steel sheet, so that the upper limit is restricted to 0.2%.

Tungsten is a heavy element which increases the specific gravity of steel so the addition, if any is within a range of 0.05 to 1.0%. Antimony (Sb) segregates on the grain boundaries restricts the formation of kappa carbide so that antimony, if added, is added in the range of 0.005% or more. However, if excessively added, antimony segregates on a grain boundary to degrade ductility, so that the upper limit thereof is restricted to 0.2%.

Ca forms sulphides such as CaS, and thereby prevents the formation of MnS, so that it is added in the range of 0.001% or more to improve hot workability of steel. The upper limit is restricted to 0.2%.

The steel sheet of the invention includes a retained austenite structure. The retained austenite complements the low strength of a ferrite matrix structure and also contributes to improvement in ductility thereof, so that it is included in the range of 5% or more by area. The upper limit thereof is preferably restricted to 20%, more preferably the upper limit is 15% or even 12%.

Hereinafter, a manufacturing method of the high strength and low specific gravity steel sheet will be described in detail.

In order to manufacture the steel sheet of the invention, a slab (i.e. a thin slab (<150 mm), thick slab (150-400 mm) or cast strip (<20 mm)) is first heated in the temperature range of 1000 to 1250° C. If the reheating temperature exceeds 1250° C., coarse grains are formed in the slab, possibly creating ridging and hot shortness, whereas if it is below 1000° C., the finishing hot-rolling temperature becomes too low to both manufacture a steel sheet and remove an oxide film on a high temperature surface using the spraying of pressurized water, thereby causing surface defects. Thus, the reheating temperature is restricted to 1000 to 1250° C. Preferably the reheating temperature is at least 1100° C.

Since the hot rolling is implemented at a temperature as low as possible so as to effectively obtain fine grains, according to the present invention, the finishing rolling is implemented at a temperature of 900° C. or less, preferably at most 850° C. in order to refine crystal grains by dynamic or static recrystallisation during the hot rolling process. This means that the material is subjected to the last hot deformation step while it is at least at the aforesaid temperature. However, if the temperature is too low, hot deformation resistance increases to make it difficult to manufacture a steel sheet, and kappa carbide is precipitated to provide elongated structures, thereby increasing ridging defects, so that the rolling temperature is in the range of 700° C. or more, preferably 750° C. or more and more preferably 800° C. or more.

The hot-rolled steel strip is coiled at a temperature of 600° C. or less. This temperature restricts the coarsening of the grain size and the excessive-precipitation of kappa carbide. It also reduces the risk of formation of abnormally coarsened grains caused by secondary recrystallisation of the coarse grains. Preferably the coiling temperature is below 550° C. The coiling temperature should be at least 200° C., and preferably at least 300° C., as quenching the material to ambient temperature proved to cause severe cracking during cold rolling.

The resulting hot-rolled material can be manufactured into a hot-rolled steel sheet after being treated with pickling, and temper rolling and oiling. According to the present invention, the steel sheet is a low density steel sheet having the specific gravity of 7400 kg/m³ or less, preferably of 7300 kg/m³ or less.

Further, the hot-rolled steel sheet can be manufactured into a cold rolled steel sheet after being pickled and cold rolled.

In the cold rolling, cold rolling reduction is set to 40% or more. This is because, if the cold rolling reduction is set to 40% or more, stored energy by cold working can be secured, and a new recrystallised structure can be obtained. Preferably the minimum cold rolling reduction is 50%. However, the upper limit thereof is restricted to 90% or less in consideration of production efficiency and economy. Optionally, the material may be subjected to intermediate annealing in between cold rolling reductions or steps.

The cold rolled steel sheet is treated with continuous annealing or continuous hot-dip galvanizing after cleaning the surface if necessary. The annealing rate is preferably selected in the range of 1° C./s to 20° C./s. If the annealing rate is less than 1° C./s, productivity is too low, and the steel sheet is exposed to high temperature condition for a long time to thereby cause the coarsening of crystal grains and reduction in strength, deteriorating the quality of material. On the other hand, if the annealing rate exceeds 20° C./s, because of insufficient re-melting of carbide, the formation of austenite also becomes insufficient and thus retained austenite is reduced to thereby reduce the ductility. The cooling rate after annealing is preferably between 10 and 50° C./s, either to ambient temperatures, or to the galvanising bath and/or the overaging treatment. After galvanising or overageing, the cooling rate is preferably between 10 and 50° C./s, more preferably between 10 and 25° C./s,

Annealing is implemented in the temperature range between the recovery temperature and 900° C. Between the recovery temperature and below the recrystallisation temperature, some ductility is recovered. This may be used to create high strength steels whilst securing adequate ductility by selecting the recovery annealing temperature and time. Above the recrystallisation temperature and below 900° C., the cold deformed structure readily recrystallises. The combination of annealing temperature and annealing time to obtain full recrystallisation of the cold-rolled steel strip can be easily determined. The inventors found a lower austenite content in the final product after annealing if the cold-rolled material was annealed at a higher annealing temperature. Above 900° C., because of the formation of a lower amount of austenite, the ductility decreases. A suitable lower limit for the annealing temperature in the case where the annealed steel was found to be 750 ° C. Preferably the lower limit is at least 800° C. The annealing is carried out for 10 seconds or more so as to achieve excellent strength and workability. However, if the annealing time exceeds 180 seconds, the productivity is excessively lowered and the properties may be adversely affected by the prolonged annealing process.

After annealing the steel sheet is cooled to the temperature of the bath and may be coated with Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, Al—Mg—Si, or the like in the thickness of 10 to 200 pm per one side thereof, thereby forming coated steel sheet, by a hot dip coating process. These or other metal coating layers may also be applied by an electroplating process. Preferably the coating thickness on the or each surface is between 10 μm and 200 μm.

Preferably the material is subjected to overageing after annealing. If applicable this overageing may precede or follow after the hot dip coating process, depending on the lay-out of the plant or depending on metallurgical preferences. The overageing temperature is preferably between 350 and 500° C., and preferably about 400° C. The overageing time is preferably at least 30 and/or at most 180 s. In case of such an overageing treatment, the annealing temperature is preferably at least 825° C. and/or preferably at most 875° C.

In the steel sheet produced as above, carbides and 5% or more of retained austenite is dispersed in a ferrite matrix, so that the tensile strength is high in the level of 600 to 900 MPa, the ductility is excellent, and therefore the combination of strength-ductility is also excellent. In a preferable embodiment the steel sheet has a tensile strength of 600 to 900 MPa.

The present invention will now be described in detail with reference to the following examples. The examples are for illustrative purposes, and are not intended to restrict the scope of the present invention in any way.

Steel slabs having compositions shown in Table 1 were produced, reheated at 1200° C. (RHT), and hot rolled at a finish rolling temperature of 900° C. The thickness of the hot-rolled steel sheet was 3 mm, and the hot-rolled steel sheet was coiled at a temperature (CT) of 400° C. or 650° C. (S3) (See Table 2). The inventors found that finish hot rolling at lower temperatures (but above 700° C.) such as 850° C. did not affect the microstructure or properties of the steel, but only had an effect on the rolling forces which increase with decreasing rolling temperatures. However, these increased rolling forces can be easily overcome by using a rolling mill of adequate power.

The steels were then subjected to cold-rolling at a reduction (CRR) of 67% and continuously annealed at annealing temperatures (AT) between 800 and 1050° C. (see Table 3) and with and without overageing at 400° C. (see Table 4). The annealing time was 60 seconds and the overageing time was 80 seconds.

TABLE 1 Chemical composition 1.7 * (Mn − Al) + No. C Al Mn Si P S N 52.7*C S1 0.18 6.7 3.4 0.01 0.004 0.003 0.005 3.90 S2 0.13 9.1 5.4 0.01 0.003 0.004 0.006 0.60 S3 0.004 6.8 0.02 0.01 0.004 0.005 0.004 −11.30 S1* 0.17 7.0 3.6 0.01 0.004 0.003 0.004 3.18 S1** 0.19 8.0 3.9 0.01 0.004 0.003 0.005 3.05

TABLE 2 Process parameters and mechanical properties (YS: yield strength in MPa, TS: tensile strength in MPa, El: elongation in %, RA: retained austenite in vol. %) RHT CT CRR AT YS TS El RA Type (° C.) (° C.) (%) (° C.) (MPa) (MPa) (%) (%) Cracking S1 1200 400 67 850 466 626 21 8 no S2 1200 400 67 850 445 675 4.5 2.5 yes S3 1200 650 67 850 342 465 31 0 no

TABLE 3 Effect of annealing temperature effect without overageing RHT CT CRR AT YS TS El RA Type (° C.) (° C.) (%) (° C.) (MPa) (MPa) (%) (%) S1 1200 400 67 800 489 652 23 10 S1 1200 400 67 850 466 626 21 8 S1 1200 400 67 900 435 652 14 6 S1 1200 400 67 1050 401 670 8 4

TABLE 4 Effect of combination of annealing temperature and overaging temperature AT OT YS TS El Type (° C.) (° C.) (MPa) (MPa) (%) S1 800 no 489 652 23 S1 800 400 489 651 23 S1 850 no 466 626 21 S1 850 400 467 631 28

Results with steels S1* and S1** were similar to those of S1 when processed like S1.

It is clear that there is always a balance between recrystallisation during annealing and the amount of retained austenite. The higher the annealing temperature, the lower the amount of retained austenite.

The overageing treatment appears to have a beneficial effect in combination with a higher annealing temperature. 

1. A low density high strength steel sheet comprising 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al and 0.01% or less N, optionally comprising one or two or more elements selected from the group consisting of 0.1% to 2.0% Si, 0.1% to 0.3% Cr, 0.05% to 0.5% Mo, 0.1% to 2.0% Ni, 0.1% to 1.0% Cu, 0.0005% to 0.003% B, 0.01% to 0.2% Ti, 0.005% to 0.2% Zr, 0.05% to 1.0% W, 0.001% to 0.2% Ca, the balance being iron and inevitable impurities, wherein 1.7·(Mn—Al)+52.7·C is at least 3 and at most 4.5.
 2. The steel sheet according to claim 1, wherein the amount of retained austenite in the sheet is 5% or more.
 3. The steel sheet according to claim 1, wherein the carbon content is at most 0.23%.
 4. The steel sheet according to claim 1, wherein the density of the steel sheet is at most 7400 kg/m³.
 5. The steel sheet according to claim 1, wherein a coating layer is provided on one or both of the major surfaces, wherein the coating layer is selected from Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, and Al—Mg—Si, and wherein the coating thickness on the or each major surface is between 10 μm and 200 μm.
 6. The steel sheet according to claim 1, wherein the value of (36·C+Mn)/Al is at least 1.3 and at most 2.0.
 7. A process for producing a low density high strength steel sheet comprising 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al, and 0.01% or less N, optionally comprising one or two or more elements selected from the group consisting of 0.1% to 2.0% Si, 0.1% to 0.3% Cr, 0.05% to 0.5% Mo, 0.1% to 2.0% Ni, 0.1% to 1.0% Cu, 0.0005% to 0.003% B, 0.01% to 0.2% Ti, 0.005% to 0.2% Zr, 0.1% to 1.0% W, and 0.001% to 0.2% Ca, the balance being iron and inevitable impurities, wherein 1.7·(Mn—Al)+52.7·C is at least 3 and at most 4.5, the process comprising the steps of: providing a steel slab, thin slab or strip having the said chemical composition; preparing the said slab or strip for hot rolling by reheating it within a range of from 1000° C. to 1250° C.; subjecting the slab or strip to the last finishing hot-rolling step while the rolled strip has a temperature of between 700° C. and 900° C.; coiling the hot-rolled strip at 600° C. or less.
 8. Process according to claim 7, the process further comprising the steps of: cold-rolling the hot-rolled steel slab or strip at a rolling reduction of 40% to 90% to produce a cold-rolled steel strip; continuously annealing the cold-rolled steel strip at a maximum temperature of 900° C. at an annealing rate of 1 to 20° C./s for 10 to 180 seconds.
 9. Process for producing a low density and high strength steel sheet according to claim 8, wherein the continuous annealing process is a recovery annealing.
 10. Process for producing a low density and high strength steel sheet according to claim 8, wherein the continuous annealing process is a recrystallisation annealing.
 11. Process according to claim 7, wherein the amount of retained austenite in the steel sheet is 5% or more.
 12. Process according to claim 7, wherein the value of (36·C+Mn)/Al is at least 1.3 and at most 2.0.
 13. Process according to claim 8, wherein after the annealing process a coating layer is provided on one or both of the major surfaces, wherein the coating layer is selected from Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, and Al—Mg—Si.
 14. Process according to claim 8, wherein the annealing process includes an overageing step.
 15. Process according to claim 8, wherein the combination of annealing temperature and annealing time is sufficient to obtain full recrystallisation of the cold-rolled steel strip.
 16. The steel sheet according to claim 1, wherein the density of the steel sheet is at most 7300 kg/m³, and wherein the steel sheet has a tensile strength of 600 to 900 MPa.
 17. Process according to claim 8, wherein after the annealing process a coating layer is provided on one or both of the major surfaces, wherein the coating thickness on the or each major surface is between 10 μm and 200 μm.
 18. Process according to claim 14, wherein the overageing step is at a overageing temperature between 350 and 500° C. for a period of between 30 to 180 seconds.
 19. Process according to claim 18, wherein the annealing temperature of the annealing process is at least 825° C. and/or at most 875° C.
 20. Process according to claim 15, wherein the annealing temperature is 750° C. or more. 